The question 'What does mainstream mean' is clearly off-topic here as general reference. But the actual implications of the way the word is used within a given domain or sub-domain (pop music) are really more domain-related than general-English related.
– Edwin AshworthJan 7 '15 at 10:41

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In fact ODO actually references music in its entry for mainstream. However, whether music can be split into kinds or remains as kind is probably a valid question if it hasn't been asked before.
– Andrew Leach♦Jan 7 '15 at 10:45

Technically, "pop" is short for "popular", and "mainstream" means, roughly, "the most common". So the term is (taken literally) kind of redundant. However, "pop" has come to mean a particular (rather broad and ill-defined) genre (that usually is assumed to exclude most "country" music, eg, even though country is, alas, quite popular). So it's quite confusing.
– Hot LicksFeb 6 '15 at 12:46

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There is not one universally agreed upon taxonomy of musical genres (though there have been efforts to create one). Most taxonomies recognize that musical genres can be classified in a hierarchical system. For example, the digital music store at Amazon.com has a top level genre (or "root genre") called Pop, and within that there are eleven sub-genres, including Dance Pop, Soft Pop, Euro Pop, Pop Rock, and Adult Alternative.